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L0301P10 - Transport Across Membranes
__TOC__ Plasma Membrane *fluid mosaic model *readily deforms and self sealing *selectively permeable (to polar solutes) *composed of lipids, proteins and some carbohydrates on the external surface *asymmetric and heterogeneous *in constant lateral motion *composition depends on the function of the membrane Functions *to compartmentalise cellular processes *organise complex reaction sequences central to: **energy conservation **cell-to-cell communication (with carbohydrates) *embedded proteins have roles in signalling, adhesion and transport Membrane Proteins Functions *pumps *energy transducers *receptors Membrane Lipids Types *phospholipids **can spontaneously form bilayers *cholesterol **found in eukaryotic cells only Structure of Phospholipids *hydrophobic fatty acid “tail” *hydrophilic phosphate “head” Structure of Fatty Acids *saturated **no double bonds in the hydrocarbon chain *unsaturated **one or more double bonds that create a kink in the chain, thus cannot be packed closely Sphingolipids *made of sphingosine, fatty acid and a carbohydrate chain *are more abundant in some parts of the plasma membrane *on the external surface of red blood cell membranes and play a role in determining blood type Transport Mechanisms ---- Passive Transport *also known as simple diffusion, diffusion *down concentration gradient *diffuse freely through the membrane **lipids **fat-soluble vitamins **steroids **water (small amounts) **gases (O2 and CO2) *occurs until equilibrium is reached ---- Transport Protein Types *Uniporter **transports one substance in one direction *Symporter **transports two different substances in the same direction *Antiporter **transports two different substances in opposite directions ---- Facilitated Transport *down concentration gradient *through a transport channel Channel Proteins *proteins with channels lined with hydrophilic amino acids so polar molecules can move through Gated Ion Channels *stimulus molecule activated by a signal e.g. high concentration of an ion outside the cell *stimulate molecules binds to the protein channel *opening them so ions (polar substances) can diffuse through the cell membrane Carrier Proteins *molecule enters carrier protein and binds to the protein *causes conformational change in the protein *allows molecule to move out the other side *carrier protein resumes original shape when molecule passes through *e.g.: glucose **via uniporter **rate of entry of glucose depends on the external glucose concentration ---- Active Transport *against concentration gradient *through a transport channel *requires ATP *different energy sources distinguish different active transport systems Primary Active Transport *involves direct hydrolysis of ATP *e.g.: sodium-potassium pump **pushes Na+ out and keeps K+ in the cell **involves ATPase, cleaves Pi from the ATP **3 Na+ binds —> ATP hydrolyses —> Na+ moves through —> change in shape of protein —> 2 K+ binds —> Pi released —> K+ moves through Secondary Active Transport *does not directly use ATP *energy is supplied by ion concentration and electrical gradient established by primary active transport **e.g.: drives glucose against gradient ***glucose entry by symport with Na+ Combined Systems *e.g.: for carbon dioxide to be converted to bicarbonate chloride ions are required **chloride taken up to convert CO2 **chloride released and CO2 exhaled ---- Transport of Large Molecules *require special mechanisms **large pores ***e.g. nuclear pores allow movement of RNA in and out the nucleus **vesicular transport (below) Exocytosis *proteins synthesised at rough ER *go through Golgi complex and packaged into a vesicle *vesicles fuse with plasma membrane Endocytosis *plasma membrane proteins *three types: **pinocytosis - engulfing liquids **phagocytosis - engulfing solids **receptor-mediated endocytosis